In recent years increased use and demand for alcohols, such as methanol, ethanol and higher alcohols has led to a greater interest in processes relating to alcohol production. The said alcohols may be produced by the fermentation of, for example, sugars and/or cellulosic materials.
Alternatively, alcohols may be produced from synthesis gas. Synthesis gas refers to a combination of hydrogen and carbon oxides produced in a synthesis gas plant from a carbon source such as natural gas, petroleum liquids, biomass and carbonaceous materials including coal, recycled plastics, municipal wastes, or any organic material. Thus, alcohol and alcohol derivatives may provide non-petroleum based routes for the production of valuable chemicals and fuels.
Generally, the production of alcohols, for example methanol, takes place via three process steps: synthesis gas preparation, methanol synthesis, and methanol purification. In the synthesis gas preparation step, an additional stage may be employed by where the feedstock is treated, e.g. the feedstock is purified to remove sulphur and other potential catalyst poisons prior to being converted into synthesis gas. This treatment can also be conducted after syngas preparation; for example, when coal or biomass is employed.
Steam reforming, for the preparation of synthesis gas, may take place in a single-step, wherein all of the energy consuming reforming reactions are accomplished in a single tubular steam reformer. However, the single-step reformer results in a production of surplus hydrogen to that required for the stoichiometry of typical alcohol(s) synthesis. For example, steam reforming of methane generally produces a syngas with a 3:1 hydrogen to carbon monoxide ratio, whereas the synthesis of methanol requires a 2:1 ratio. Indeed, ethanol synthesis catalysts prefer to operate at a 1:1 ratio of hydrogen to carbon monoxide. Consequently, when using methane as a feedstock, the excess hydrogen (in terms of the typical required molar ratio for higher alcohol synthesis) must be utilised efficiently, and hence an additional separation stage is often employed, the recovered hydrogen may then be used as a fuel or as a reagent in another chemical process, the hydrogen recovery process causes a substantial increase to the overall expenditure of the process.
Alternatively, the synthesis gas preparation may take place in a two-step reforming process wherein the primary reforming in a tubular steam reformer is combined with an oxygen-fired secondary reforming step which produces a synthesis gas with a deficiency in hydrogen for the stoichiometry required for typical alcohol(s) synthesis. With this combination it is possible to adjust the synthesis gas composition to obtain the most suitable composition for methanol synthesis.
As an alternative, auto-thermal reforming, wherein a stand-alone, oxygen-fired reformer produces synthesis gas having a hydrogen deficiency followed by the downstream removal of carbon dioxide to restore the desired ratio of hydrogen to carbon oxide, results in a simplified process scheme. However, when using an auto-thermal reformer to generate syngas to produce C2 and C2+ alcohol(s), it is has been found necessary to import a separate feedstock of CO2 to the feed prior to it entering into the oxygenate synthesis reactor in order to achieve the optimum syngas molar ratio of (H2-CO2):(CO+CO2) and thereby increasing the expenditure and CO2 emissions of the overall process.
The reaction to produce alcohol(s) from syngas is generally exothermic. The formation of C2 and C2+ alcohols is believed to proceed via the formation of methanol for modified methanol catalysts and cobalt molybdenum sulphide catalysts. However, the production of methanol is equilibrium limited and thus requires high pressures in order to achieve viable yields. Hence, pressure can be used to increase the yield, as the reaction which produces methanol exhibits a decrease in volume, as disclosed in U.S. Pat. No. 3,326,956. Improved catalysts have now allowed viable rates of methanol formation to be achieved at reduced reaction temperatures, and hence allow commercial operation at lower reaction pressures, e.g. a copper oxide-zinc oxide-alumina catalyst that typically operates at a nominal pressure of 5-10 MPa and temperatures ranging from approximately 150 DEG C. to 450 DEG C. over a variety of catalysts, including CuO/ZnO/Al2O3, CuO/ZnO/Cr2O3, ZnO/Cr2O3, and supported Fe, Co, Ni, Ru, Os, Pt, and Pd catalysts. A low-pressure, copper-based methanol synthesis catalyst is commercially available from suppliers such as BASF, ICI Ltd. of the United Kingdom, and Haldor-Topsoe. Methanol yields from copper-based catalysts are generally over 99.5% of the converted CO+CO2 present. Water is a by-product of the conversion of CO2 to methanol and the conversion of CO synthesis gas to C2 and C2+ oxygenates. In the presence of an active water gas-shift catalyst, such as a methanol catalyst or a cobalt molybdenum catalyst the water equilibrates with the carbon monoxide to give CO2 and hydrogen. A paper entitled, “Selection of Technology for Large Methanol Plants,” by Helge Holm-Larsen, presented at the 1994 World Methanol Conference, Nov. 30-Dec. 1, 1994, in Geneva, Switzerland; reviews the developments in methanol production and shows how further reduction in costs of methanol production will result in the construction of very large plants with capacities approaching 10,000 metric tonnes per day.
Other processes, for the production of C2 and C2+ alcohol(s), include the processes described hereinafter; U.S. Pat. No. 4,122,110 relates to a process for manufacturing alcohols, particularly linear saturated primary alcohols, by reacting carbon monoxide with hydrogen at a pressure between 20 and 250 bars and a temperature between 150 DEG and 400 DEG C., in the presence of a catalyst, characterized in that the catalyst contains at least 4 essential elements: (a) copper (b) cobalt (c) at least one element M selected from chromium, iron, vanadium and manganese, and (d) at least one alkali metal.
U.S. Pat. No. 4,831,060 relates to the production of mixed alcohols from carbon monoxide and hydrogen gases using a catalyst, with optionally a co-catalyst, wherein the catalyst metals are molybdenum, tungsten or rhenium, and the co-catalyst metals are cobalt, nickel or iron. The catalyst is promoted with a Fischer-Tropsch promoter like an alkali or alkaline earth series metal or a smaller amount of thorium and is further treated by sulfiding. The composition of the mixed alcohols fraction can be selected by selecting the extent of intimate contact among the catalytic components.
Journal of Catalysis 114, 90-99 (1988) discloses a mechanism of ethanol formation from synthesis gas over CuO/ZnO/Al2O3. The formation of ethanol from CO and H2 over a CuO/ZnO methanol catalyst is studied in a fixed-bed microreactor by measuring the isotopic distribution of the carbon in the product ethanol when 13C methanol was added to the feed.
At present, there are two major issues, associated with the conversion of hydrocarbons to alcohol(s), which need to be addressed.
The first issue is primarily an environmental concern, as when manufacturing and using syngas as part of an integrated process, the high temperatures necessary for syngas formation, are often generated by the burning of carbonaceous fuel, and hence dilute carbon dioxide is produced as a result.
In addition to this, is the fact that water is produced as a result of the conversion of syngas to C2 and C2+ alcohol(s), which consequently is rapidly converted to carbon dioxide and hydrogen during the oxygenate synthesis reaction stage due to the nature of the typical catalysts used (i.e. active water gas-shift catalysts), and the reaction conditions typically employed in these processes. Hence, during the overall integrated process, significant amounts of carbon dioxide is produced and emitted into the environment.
Contributing significantly to the above issue, is the factor that the use of a typical higher alcohol catalysts during the alcohol synthesis stage of the process, results in the build up of alkanes (due to loss of selectivity) during the required gas recycling process, which will often necessitate in a purge. Typically purge streams are fuelled or flared which can add significant amounts of CO2 to the overall carbon emissions.
The level of carbon dioxide present in the atmosphere is a well documented environmental concern of today's world, as carbon dioxide is considered to be the most prominent of all the ‘greenhouse gases’, and therefore one of the main pollutants in the present atmosphere. For this reason, it is of global interest and concern to reduce carbon dioxide emissions in industrial processes to a minimum as far as possible.
The second issue associated with the conversion of hydrocarbons to alcohol(s), wherein the hydrocarbons are first converted into syngas, which is then subsequently converted into alcohol(s), is concerned with the overall heat efficiency of the process. Historically, there has been a lot of interest shown in trying to utilise the heat generated during the syngas to alcohol(s) conversion stage efficiently, for example by generating steam from the excess heat and subsequently using the steam to provide the energy to power alternative processes. However, depending on the production site in question, this is not always possible. Therefore, recently more interest has been shown in trying to improve efficiency of the heat generated in a more local manner, i.e. as part of an integrated process.